Bitops and arrays

I started working on porting [datascript](https://github.com/tonsky/datascript) to pixie. I've started with the `datascript.btset` namespace there and it needs the `bit-*` functions and arrays.

I'm not completely happy with duplicating `nth` and `count` in `aget` and `alength`, but I'd also like to keep the changes to datascript minimal.

Before this gets merged I'd like to add a few tests.